Composite heat exchanger shell and buoyancy system and method

ABSTRACT

A heat exchanger includes a shell made of a composite material, and a heat exchanger housed substantially within the shell. The shell is made of a composite material further comprises planks positioned in the outer periphery of the shell. The planks, in one embodiment, are substantially hollow or include substantially hollow portions. In some embodiments, the planks are formed of pultruded plastic. The shell of the heat exchanger further includes layers of fiberglass. The pultruded plastic planks are sandwiched between at least a first layer of fiberglass and a second layer of fiberglass. The layers of fiberglass are infused with resin. A floating portion of an Ocean Thermal Energy System includes shells made of composite material. The cold seawater intake can also be an elongated tube of composite material.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/570,733, filed on Dec. 14, 2011, which is incorporated by reference herein.

TECHNICAL FIELD

Various embodiments described herein relate to a composite heat exchanger shell and buoyancy system and method. The composite heat exchanger shell and buoyancy system and method is used as part of a floating Ocean Thermal Energy Conversion (“OTEC”) system. It can also be used in floating thermal desalination plants and in other marine and land-based applications.

BACKGROUND

An increase in worldwide population has led to the increase in demand for fresh water for human consumption and irrigation. Over 99% of the world's fresh water comes from tapping a diminishing source of the world's rivers, lakes, and groundwater locations that are becoming less dependable as some are reaching maximum capacities. With only 1% of the world's water supply available for human use in a constantly expanding worldwide population, clean water is becoming the most important commodity in water-stressed regions. The increase in demand for fresh water has been most evident in dry areas where rainwater is scarce and groundwater sources are drying up such as: the Middle East, Australia, and the American West and Southwest, to name a few.

Clean water is necessary for irrigation in arid regions where occupants rely on importing most of their food because agriculture is too expensive or not possible. Although clean water is basic utility in water-rich and developed regions, the arid and less developed regions of the world do not have access to clean water.

Most of the earth's surface, about 71%, is covered with water. However, most of the water is in saltwater oceans. Of course, salt water is unfit for human consumption. Water can be desalinated. The two most common options for water production include non-thermal/pressure/membrane processes, and thermal processes. The non-thermal/pressure/membrane processes include reverse osmosis (“RO”), filtration, sludge, and the like. The thermal processes include multi-stage flash, multi-effect distillation, and low-temp thermal desalination. Generally, water treatment and desalination methods require capital intensive equipment and facilities that become more expensive in regions that are arid and underdeveloped.

Each of the thermal processes includes a heat exchanger which is generally used to transfer heat from steam or humid air to cooler seawater. By transferring heat from the steam or humid air, freshwater condenses onto the heat transfer surfaces of the heat exchanger. As in the case of OTEC, expensive materials drive up heat exchanger capital costs and often eliminate the thermal desalination process from consideration.

Large heat exchangers are required for Ocean Thermal Energy Conversion (OTEC) for producing power based on the temperature difference between deep seawater and seawater near the surface of the ocean. A closed Rankine cycle using ammonia as the working fluid is commonly used in OTEC. Warm seawater is used to transfer heat to the boil liquid ammonia in the evaporator of the Rankine cycle. The cold seawater is used to remove heat from ammonia gas during a substantially constant pressure transfer of heat from the ammonia gas as it condenses in the condenser. Both the evaporator and the condenser each comprise one or more heat exchangers.

Expensive corrosion resistant metals are normally required for these heat exchangers since sea water is corrosive. The ammonia working fluid is used in the discussed application, but is incompatible with alloys containing copper, and titanium has been cited as the baseline material in past studies for OTEC plants but this idea is not restricted to ammonia as the working fluid. Many of the heat exchangers employ shell and tube technology; while others incorporate more compact plate-fin geometries. The expensive materials drive up the capital expenditure associated with OTEC heat exchangers to largely restrict locations where plants can be economically deployed.

Aluminum tubes or extrusions can be used in the heat exchangers. However, when a different metal is used as an exposed end plate or sheet in these heat exchangers a galvanic reaction causes corrosion concerns. Even with an aluminum tubesheet, the traditional fusion welding process (MIG or TIG welding) requires a filler material with a different alloy composition than the base metal. Fusion welding also results in a heat affected zone, where the region around the weld joint has a different grain structure than the surrounding tube or tube sheet material. These negative impacts from fusion welding can produce accelerated corrosion. Other joining techniques introduce aluminum alloys with a different composition than the base metal to weld or braze aluminum tubes or extrusions to adjacent aluminum plates. This too causes corrosion problems based on the formation of a galvanic reaction. In many heat exchangers the aluminum tubes must be isolated from a sheet of dissimilar material by way of an isolating gasket. Even with the gasket isolating dissimilar metals from each other, crevices at the tube end/tube sheet joint can trap chloride ions, resulting in preferential corrosion. Even if a bundle of aluminum tubes can be attached to an aluminum sheet without using a different material, these condensers are generally housed in a structural steel shell. Aluminum sheets and tubes must be isolated from the steel shell to prevent or substantially reduce a galvanic corrosion reaction.

Non-OTEC solutions for production of power are generally land based where weight of the heat exchangers can be easily accommodated. However, economical floating OTEC plants typically require buoyant structures to support the heat exchangers near the surface of the ocean.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a Rankine cycle that uses ammonia as a working fluid to generate power, according to an example embodiment.

FIG. 2 is a schematic view of a floating portion of a plant, according to an example embodiment.

FIG. 3 is a perspective view of the outlet end of the cold water pipe which has a plurality of shells attached thereto, according to an example embodiment.

FIG. 4 is a set of photos showing one or more aluminum tubes friction stir welded to a tube sheet to form the core of a heat exchanger, according to one example embodiment.

FIG. 5 is a set of photos showing one or more multi-hollow aluminum extrusions friction stir welded (FSW) to a tube sheet, according to another example embodiment.

FIG. 6 shows an exploded view of a heat exchanger positioned within a composite heat exchanger shell, according to another example embodiment.

FIG. 7 shows a perspective cutaway view of a composite heat exchanger shell 640, according to an example embodiment.

FIG. 8 shows a cutaway view of a flange attached to an end of a composite heat exchanger shell, according to an example embodiment.

FIG. 9 is a side cut-away view of an apparatus for forming a composite shell on a platform, according to an example embodiment.

FIGS. 10A-E depict various portions of the apparatus shown schematically in FIG. 9.

DETAILED DESCRIPTION

FIG. 1 is a schematic view of a Rankine cycle that uses ammonia as a working fluid to generate power, according to an example embodiment. The Rankine cycle 100 includes an evaporator 110, a turbine 120, a condenser 130, and a pump 140. The evaporator 110, the turbine 120, the condenser 130, and the pump 140 are in fluid communication with one another. Ammonia in a liquid phase and a vapor phase is circulated to the various components of the Rankine cycle 100. Fluid lines join the evaporator 110 to the input of the turbine 120 and the fluid output of the turbine 120 to the condenser 130. The fluid output of the condenser 130 is attached to the pump 140. As mentioned, in this embodiment ammonia is the working fluid but various other working fluids can be used. Large flows of warm surface seawater are used to boil the ammonia in evaporators or evaporator 110. The ammonia vapor drives the turbine 120. Attached to the turbine is a rotor portion generator 122. Rotating the shaft of the turbine 120 rotates the rotor of a generator to generate electric power. To complete the cycle, low pressure ammonia vapor exits the turbine 120 and enters condenser 130 or condensers which are cooled using large flows of cold, deep ocean water. A pump 140 add pressure to the condensed ammonia liquid at the output of the pump 140 to match the required input pressure of the evaporator 110. It should be noted that there may be more than multiple components of the evaporator 110, the turbine 120, the condenser 130 and the pump 140 in a given OTEC plant. Other pumps are used to move the warm and cold seawater through the evaporator 110, and the condenser 130, respectively. An OTEC plant requires no fuel and has low operating costs (a portion of the gross generated power is assigned to pump the warm and cold seawater).

FIG. 2 is a schematic view of a floating portion of a plant 200, according to an example embodiment. The floating portion of the plant can be for an OTEC plant or for a desalination plant or any other plant that would require heat exchangers of this nature. For the sake of convenience, an OTEC plant will be further detailed initially. The floating portion 200 includes a ship (barge) 210 and a floating condenser portion 220. The ship 210 in the OTEC plant includes the evaporator 110, the turbine 120, and the generator 122 for producing power. The condenser portion 220 includes a buoyancy section 222, a plenum 224, a platform 226, and a cold water pipe 228. The buoyancy section 222 is a watertight chamber or substantially watertight chamber that is of sufficient space to provide a buoyancy force to hold the remaining portions of the floating condenser portion 220. Lesser weight of the components results in a smaller chamber. Considerable weight can be saved by making the heat exchanger 130, 230 from aluminum (as discussed below) and by making a shell 240 to enclose the heat exchanger out of a composite material. Of course, the smaller the chamber or buoyancy section 222, the less material needed and the less cost associated with the floating condenser portion 222 of the OTEC plant. Attached to the top of the buoyancy section 222 is a platform 226. The platform 226 provides routing for cables and pipes or lines connected between the ship 210 and the condensing portion 220. The platform 226 also includes a flat surface on which helicopters can land or on which people can place instruments or tools. The platform 226 can also serve as an area on which to store machines and materials for producing some of the parts of the condensing portion 220. Attached below the buoyancy section 222 is a plenum 224. The plenum 224 is a pressurized housing containing a fluid, in this case cold seawater, at a positive pressure. The cold water pipe 228 is attached to the plenum 224. The pressure in the plenum 224 is higher than the pressure of the surroundings. The plenum 224 equalizes the pressure for more even distribution of the cold seawater, because of irregular supply or demand. The cold water pipe 228 is in fluid communication with the plenum 224. The cold water pipe 228 is very long and reaches down well below the thermocline. The distances can be 300 meters to 1000 meters or more. The cold water pipe 228 has a length to extend down to water having a temperature that will condense ammonia vapor within the at least one condenser 130 (see FIG. 1). The condenser portion 220 also includes at least one condenser 130. The condenser includes at least one heat exchanger 230 housed within a shell 240. In one embodiment, the shell 240 also includes an axial pump 250 on the outlet side of the heat exchanger. The inlet side of the heat exchanger 230 is positioned near the cold water inlet end 241 of the shell 240. The cold water inlet end 241 of the shell 240 is in fluid communication with the plenum 224. The axial pump 250 is positioned near the cold water outlet end 242 of the shell 240. The axial pump 250 moves the water through the heat exchanger 230, the plenum 224 and up the cold water pipe 228.

FIG. 3 is a perspective view of the outlet end 229 of the cold water pipe 228 which has a plurality of shells 240 attached thereto, according to an example embodiment. The buoyancy section 222, the plenum 224, and the platform 226 are not shown in FIG. 3. As shown, there are a plurality of shells 240 attached near or proximate the cold water outlet or discharge end 229 of the cold water pipe 228. In FIG. 3, there are eight shells attached to the cold water pipe 228. It should be noted that more or even a lesser number of shells 240 can be attached to the cold water pipe 228. The shells are positioned around the central cold water pipe 228 in the embodiment shown. The plurality of shells 240 are attached to a first structure 340 about midway down the length of the plurality of shells 240. The plurality of shells 240 are attached to a second structure 342 near the discharge end 242 of the plurality of shells 240. The first structure 340 and the second structure 342 hold the shells 240 about the outlet end 229 of the cold water pipe 228. In one embodiment, each of the shells includes an axial pump 250 and a heat exchanger 230. The heat exchangers 230 are used to condense ammonia vapor to a liquid state in an OTEC system.

In operation, the plurality of axial pumps 250 located in the plurality of shells 240 essentially pump cold water from the inlet end of the cold water pipe 228 up from depths of 1000 meters. The pumps 250 move the cold water into the plenum 224 and through the heat exchangers 230 in the shells 240. The heat exchangers 230 are used to change ammonia gas to ammonia liquid in an OTEC system.

As shown in FIGS. 2 and 3, the condensing portion 220 of the OTEC plant 200 floats in the water, separate from the ship or barge 210. In order to lower the cost of OTEC solutions, the components are made of lighter materials. Two of the components, the heat exchanger 230 and the shell 240 include lighter weight materials than steel or titanium metals which were used in the past. In the embodiment shown, the interior (core) of heat exchangers 230 within the shells are made of aluminum. Incorporating aluminum core construction within heat exchangers 230 brings substantial reduction in capital cost. For example, an aluminum heat exchanger 230 is roughly half the heat exchanger cost compared to a heat exchanger made of titanium. Further weight and cost reductions are achieved by forming the exteriors shells of heat exchangers 230 from composite materials, as well as the plurality of structural shells 240 from composite materials rather than steel or other heavy metal shells. Reductions such as these make OTEC solutions more cost-competitive in more geographic locations.

FIG. 4 is a set of photos showing one or more aluminum tubes friction stir welded (FSW) to a tube sheet, according to one example embodiment. Aluminum tube and sheet heat exchangers are fabricated using a friction stir welding (FSW) approach. Using the FSW approach, a bundle of tubes or extrusions can be joined to an aluminum tube sheet in such a way that there is a minimal heat affected zone and no dissimilar metal at the joints. Furthermore, there are no crevices to act as sites for corrosion initiation in flowing seawater. With the FSW approach, aluminum heat exchanger cores can serve as a low cost alternative to titanium and high alloy stainless steel construction. In addition to this fabrication technique, the aluminum heat exchangers are provided with graphite foam or compact aluminum fin heat transfer surfaces to enhanced the heat transfer capability of the heat exchangers 230.

FIG. 5 is a set of photos showing one or more multi-hollow aluminum extrusions 510 friction stir welded (FSW) to a tube sheet, according to another example embodiment. The aluminum extrusion 510 shown includes graphite foam attached to the multi-hollow extrusion 510. FIG. 5 also includes a cross sectional view of aluminum seawater passageways 520, 522, 524 which sandwich several layers 530, 532 of graphite foam. The graphite foam enhances the heat transfer characteristics of the resultant heat exchanger. Other materials such as metal foams and compact aluminum fins can be used as alternatives to the graphite foam.

FIG. 6 shows an exploded view of a heat exchanger 230 positioned within a composite heat exchanger shell 640. The shell 640 is a lightweight, low cost, low pressure, fiber composite shell for use in OTEC applications, desalination applications, marine heat exchanger applications, and other applications. Inside the composite shell 640, one or more aluminum cores or heat exchangers 230 comprising tube bundles or extrusion are installed. In the embodiment shown, the tubes or extrusions are joined into circular tube sheets using Friction Stir Welding (FSW). The tube bundles or extrusions can be plain aluminum. In some embodiments, the tube bundles or extrusions include graphite foam bonded to the outside tube or extrusion surfaces. In other embodiments, compact aluminum fins are bonded or brazed to the extrusions in place of graphite foam. The graphite foam or bonded/brazed aluminum fins enhances the thermal transfer characteristics of the heat exchanger 230. Using a fiberglass or composite shell 640 to enclose a substantially aluminum heat exchanger core can produce lower thermal energy input requirements, decreased cost of capital equipment, decreased buoyancy requirements, and increased resistance to corrosion and other desirable results. As shown in FIG. 6, the shell 640 also includes a first pipe flange 810, and a second pipe flange 810′.

In the marine HX application, seawater (first fluid) flows axially through the inside of tubes or extrusions. Other corrosive or non-corrosive fluids can flow through the tubes, depending on the application. On the shell side of the heat exchanger (ie outside of the tubes or extrusions), a second fluid absorbs heat from or rejects heat to the surfaces on outsides of tubes or extrusions. This shell-side fluid can be a gas, liquid or two-phase (boiling or condensing). The fluid can be ammonia, water/water vapor or other liquids and gases depending on the application. The shell-side fluid enters/exits via side ports, or through co-axial ports at the ends of HX.

FIG. 7 shows a perspective cutaway view of a composite heat exchanger shell 640, according to an example embodiment. The composite heat exchanger shell 640 includes hollow planks 710. In one embodiment, the hollow planks 710 are also made of fiberglass or another composite. In still another embodiment, the planks are pre-pultruded hollow planks 710. The hollow planks 710 are surrounded by fabric which is longitudinally continuous so as to form a substantially continuous inside fabric layer 720 and a substantially continuous outside fabric layer 722. The fabric layers 720, 722 are infused with a vinyl ester resin. In short, the composite heat exchanger shell adopts a tube architecture, utilizing sandwich wall construction. The pre-pultruded hollow planks 710 are assembled into core rings. Face sheets of longitudinally continuous fabric are applied over the assembled core rings. Face sheet material consists of low-cost glass fibers with excellent fatigue resistance. The face sheets and the core planks are joined together, and a vinyl ester resin is infused using a Vacuum Assisted Resin Transfer Molding (VARTM) process.

FIG. 8 shows a cutaway view of a flange 810 attached to an end of a composite heat exchanger shell 640, according to an example embodiment. The flange includes an inner termination ring 820 and an outer termination ring 830. The inner termination ring 820 includes grooves 822. The outer termination ring 830 includes grooves 832. The traplock grooves 822, 832 transfer axial tensile and compression loads from individual fabric plies to the metallic termination rings 820, 830 which form the flange 810. The inner termination ring 820 and the outer termination ring 830 are substantially continuous. The inner termination ring 820 and the outer termination ring 830 are tied together with bolts, such as bolts 841, 842, 843, 844. The flange 810 works for moderate pressure applications (˜15-300 psi). In another embodiment, adhesive bonded rings and a circumferential clamp can be used. For example, in one embodiment, a marlin clamp is used.

FIG. 9 is a side cut-away view of an apparatus 1200 for forming a composite shell 640 on a platform, such as platform 1202, according to an example embodiment. The apparatus 1200 is used to fabricate composite articles, such as the shell 630 or even a cold water pipe 228. Apparatus 1200 is generally suitable for fabricating continuous-fiber composite articles and is uniquely well suited for fabricating multi-shot, continuous-fiber composite articles, especially those that are very wide and very tall.

In the illustrative embodiment, apparatus 1200 is disposed on floating platform 1202, which, in some embodiments, ultimately serves as a part of an OTEC plant (see FIG. 2). Apparatus 1200 is oriented vertically on platform 1202; that is, the axial (as opposed to radial) direction of a work piece produced via apparatus 1200 is vertically aligned.

Apparatus 1200 comprises fiber supply region 1206 and molding region 1212. Fiber supply region 1206 provides a continuous supply of fiber to the molding region. The fiber used in continuous-fiber composite materials is typically available in a variety of forms, including uni-directional tapes of various widths, plain weave fabric, harness satin fabric, braided fabric, and stitched fabric. Commonly-used fibers include, without limitation, fiber glass, commercially available from Owens Corning Technical fabrics, PPG, AGY and carbon fiber, commercially available from Zoltek and others. For use in conjunction with the present invention, the fiber is typically in the form of a fabric, provided in a convenient width as a function of the intended cross-sectional shape and size of the article (e.g., 1 to 2 meters width for a 10-meter diameter pipe, etc.). Such fabrics as fiberglass are, and as carbon fibers from Zoltek and others.

Fiber supply region 1206 and molding region 1212 are environmentally isolatable, collectively, from the other regions of apparatus 1200. This is illustrated by notional access-way 1207 in fiber supply region 1206 and a seal at the bottom of molding region 1212. The access-way is required to enable core 1214, discussed further below, to be inserted into molding region 1212.

In the illustrative embodiment, fiber in the form of fabric 1210A and 1210B (collectively “fabric 1210”) is disposed on respective rolls 1208A and 1208B (collectively “rolls 1208”). Rolls 1208A and fabric 1210A are disposed radially-outward of rolls 1208B and fabric 1208B. In the illustrative embodiment, there is no difference in material type between fabric 1210A and 1210B. In accordance with the present invention, continuity of fiber is maintained between fabric 1210 in supply region 1206 and fabric 1210 that has been fed to molding region 1212.

The inner portions of apparatus 1200, such as inner fabric rolls 1208B and central inner shell 1213 are stabilized/supported via vertical central member 1205. The central member is, in turn, supported by frame 1204.

In the illustrative embodiment, core 1214 is disposed in molding region 1212. The core material, which in the illustrative embodiment is available as a plurality of plank-like segments, forms a cylindrical shape or ring when assembled and positioned in molding region 1212. This core ring (cylindrical or otherwise) establishes the basic shape for the work piece being produced in molding region 1212. As depicted in FIG. 9, fabric 1210 is disposed on both sides of core 1214 in preparation for fabricating a work piece. More particularly, fabric 1210A is disposed between core 1214 and the outer circumference of molding region 1212 and fabric 1210B is disposed between core 1214 and the inner circumference of molding region 1212. As described in further detail later in this specification, the process proceeds by compacting fabric 1210 on both sides of core 1214 against the core, infusing the fabric with resin, and then curing the resin.

In the illustrative embodiment, core 1214 is lowered into molding region 1212 via overhead traveling crane 1203. In some embodiments, the core comprises hollow planks produced from fiber and polymer via a pultrusion process, some of which may be available at a cost per pound which is generally low compared to other methods of fabricating linear composite shapes, from pultruders such as Glasforms and Strongwell. Other processes can be used to produce a structure suitable for use as core 1214. In some other embodiments, the core can be produced from other materials (e.g., aluminum, etc.) and exhibit other structural arrangements (e.g., foam, sealed honeycomb internal arrangement, etc.).

FIGS. 10A-E depict various portions of the apparatus 900 discussed above. FIG. 10A shows the shear key and core assembly, FIGS. 10B and 10C show the fabric dispensing and guidance system, and FIGS. 10D and 10E show the pipe molding region for forming the composite heat exchanger shell 240, 640.

Up to this point, a substantially aluminum heat exchanger 230 enclosed within a composite shell 640 has been discussed with respect to use in an OTEC application. It should be noted that a substantially aluminum heat exchanger 230 enclosed within a composite shell 640 can also be used as a condenser for desalination projects. The desalination projects can be for removing water from ambient air in humid climates or can be for desalination projects that remove water that use energy to flash or boil water off from seawater, such as multi stage flash (“MSF”) or multi effect distillation (“MED”). The desalination projects could even be conducted in tandem with a project, such as the production of power, that has waste heat generated. It should be noted that desalination projects represent another capital intensive market which can benefit from innovative, low cost heat exchanger/condenser solutions.

A heat exchanger includes a shell made of a composite material, and a heat exchanger housed substantially within the shell. The shell is made of a composite material further comprises planks positioned in the outer periphery of the shell. The planks, in one embodiment, are substantially hollow or include substantially hollow portions. In some embodiments, the planks are formed of pultruded plastic. The shell of the heat exchanger further including layers of fiberglass. The pultruded plastic planks are sandwiched between at least a first layer of fiberglass and a second layer of fiberglass. The layers of fiberglass are infused with resin. In one embodiment, the layers of fiberglass are infused with resin using a Vacuum-Assisted Resin Transfer Molding (VARTM) process. In addition, the shells of the heat exchanger includes a first end and a second end. At least one metal flange is coupled to one of the first end or second end of the shell. The metal flange includes an inner termination ring having inner trap-lock grooves therein, and an outer termination ring having inner trap-lock grooves therein. The shell of composite material can be formed to be buoyant. The shells can be long. For example, the shell can be over 70 meters in length. In some embodiments, the shells can be over 100 meters in length.

An apparatus for forming elongated tubes of composite material includes a floating base, a molding region attached to the base, a fiber supply region, and a resin infusion apparatus for infusing the fiber with resin. The molding region including a core ring for receiving a core material. The fiber supply region provides a substantially continuous supply of a fiber to the molding region. The fiber supply region supplies fiber to an inner region of the core ring and an outer region of the core ring. The apparatus for forming elongated tubes also includes a supply of a core material that is inserted into the core ring. A plurality of elongated planks are positioned around the ring core in forming the elongated tubes. The core material is inserted into the core ring, the core material including elongated planks of plastic. In one embodiment, the core material includes elongated planks of pultruded plastic having hollow portions therein. A vacuum assisted resin infusion apparatus can also be added to the apparatus to apply a vacuum to a portion of the molding chamber. In one embodiment, The apparatus for forming elongated tubes of composite material wherein the molding process is performed at a site where the elongated tubes of composite material are used to form a floating heat exchanger for an Ocean Thermal Energy System. The apparatus for forming elongated can also include apparatus for applying a metal flange to at least one of the ends of the elongated tube.

A floating portion of an Ocean Thermal Energy System includes a platform, a plenum attached to the platform, a cold seawater intake attached to the plenum, and a plurality of heat exchangers attached to the plenum. The heat exchangers include pumps for moving the cold seawater from the plenum through the heat exchangers. The heat exchangers include shells made of composite material. The cold seawater intake can also be an elongated tube of composite material.

A heat exchanger shell can comprise composite sandwich wall construction. The wall, in one embodiment, include pultruded plastic core “planks” sandwiched between layers of fiberglass, infused with resin. The process used to infuse the fiberglass with resin can be Vacuum-Assisted Resin Transfer Molding (VARTM). This type of construction can be used to form The heat exchanger shell can be produced using a vertical molding apparatus). It is contemplated that the heat exchanger shell can also be produced using a horizontal molding apparatus. In one embodiment, the molding apparatus can be used to form shells on site, such as on a platform floating in the ocean during construction of an OTEC system, such as shown in FIG. 2 above. Shells or composite tubes formed as above can be very long, and continuous structures measuring up to 100 m or greater in length, and up to 10 m in diameter.

Metal flanges can be attached to the ends of the composite shells or tubes. The metal flanges are ASME Code-compliant metalic flanges made of aluminum, steel, stainless steel or other metals, The metal flanges are attached to the ends of the composite shell using a fiber entrapment approach.

Condenser Heat Exchangers with composite shell can be made buoyant in water by adjusting the tube-tube pitch, tube wall thickness and number of tubes in tube bundle, to form a buoyant structure. The combined operating weight (dry weight plus water in tubes and condensing vapor outside of tubes, plus the composite shell weight) is less than the weight of surrounding water that is displaced. As a result, the floating portion 200 floats, as shown in FIG. 2.

The buoyant composite shell heat exchanger can support other elements in a floating OTEC or desalination plant, such as a central cold water pipe/riser (used to pump cold, deep ocean water to the OTEC plant), or the OTEC platform containing power generation and transmission equipment.

The buoyancy of composite shell heat exchanger can be adjusted in-situ by controlling the level of liquid (i.e. ammonia or other working fluid) on the outside of heat exchanger tubes (surrounded the composite shell).

This has been a detailed description of some exemplary embodiments of the invention(s) contained within the disclosed subject matter. Such invention(s) may be referred to, individually and/or collectively, herein by the term “invention” merely for convenience and without intending to limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. The detailed description refers to the accompanying drawings that form a part hereof and which shows by way of illustration, but not of limitation, some specific embodiments of the invention, including a preferred embodiment. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to understand and implement the inventive subject matter. Other embodiments may be utilized and changes may be made without departing from the scope of the inventive subject matter. Thus, although specific embodiments have been illustrated and described herein, any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 

What is claimed:
 1. A heat exchanger comprising: a shell made of a composite material; and a heat exchanger housed substantially within the shell.
 2. The heat exchanger of claim 1 wherein the shell of composite material further comprises planks positioned in the outer periphery of the shell. The heat exchanger of claim 2 wherein the planks are substantially hollow.
 3. The heat exchanger of claim 2 wherein the planks are formed of pultruded plastic.
 4. The heat exchanger of claim 1 wherein the composite material is further comprised of: pultruded plastic core planks; and layers of fiberglass, the pultruded plastic planks sandwiched between at least a first layer of fiberglass and a second layer of fiberglass.
 5. The heat exchanger of claim 1 wherein the layers of fiberglass are infused with resin.
 6. The heat exchanger of claim 5 wherein the layers of fiberglass are infused with resin using a Vacuum-Assisted Resin Transfer Molding (VARTM) process.
 7. The heat exchanger of claim 1 wherein the shell further comprises: a first end; a second end; and at least one metal flange coupled to one of the first end or second end of the shell.
 8. The heat exchanger of claim 7 wherein at least one metal flange is comprised of: an inner termination ring having inner trap-lock grooves therein; and an outer termination ring having inner trap-lock grooves therein.
 9. The heat exchanger of claim 1 wherein the shell of composite material is buoyant.
 10. The heat exchanger of claim 1 wherein the shell of composite material is over 70 meters in length.
 11. An apparatus for forming elongated tubes of composite material, the apparatus comprising: a floating base; a molding region attached to the base, the molding region including a core ring for receiving a core material; a fiber supply region for providing a substantially continuous supply of a fiber to the molding region, the fiber supply region supplying fiber to an inner region of the core ring and an outer region of the core ring; and a resin infusion apparatus for infusing the fiber with resin.
 12. The apparatus for forming elongated tubes of composite material of claim 11 further comprising a core material that is inserted into the core ring.
 13. The apparatus for forming elongated tubes of composite material of claim 11 further comprising a core material that is inserted into the core ring, the core material including elongated planks of plastic.
 14. The apparatus for forming elongated tubes of composite material of claim 11 further comprising a core material that is inserted into the core ring, the core material including elongated planks of pultruded plastic having hollow portions therein.
 15. The apparatus for forming elongated tubes of composite material of claim 11 further comprising a vacuum assisted resin infusion apparatus to apply a vacuum to a portion of the molding chamber.
 16. The apparatus for forming elongated tubes of composite material of claim 11 wherein the molding process occurs at a site where the elongated tubes of composite material are used to form a floating heat exchanger for an Ocean Thermal Energy System.
 17. The apparatus for forming elongated tubes of composite material of claim 11 wherein a plurality of elongated planks are positioned around the ring core in forming the elongated tubes.
 18. The apparatus for forming elongated tubes of composite material of claim 11 further comprising an apparatus for applying a metal flange to at least one of the ends of the elongated tube.
 19. A floating portion of an Ocean Thermal Energy System comprising: a platform; a plenum attached to the platform; a cold seawater intake attached to the plenum; a plurality of heat exchangers attached to the plenum, the heat exchangers including pumps for moving the cold seawater from the plenum through the heat exchangers, wherein the heat exchangers include shells made of composite material.
 20. A floating portion of an Ocean Thermal Energy System of claim 19 wherein the cold seawater intake is an elongated tube of composite material. 